A typical dual-spindle machining apparatus has a frame defining first and second upper machining stations horizontally offset from each other and first and second lower transfer stations underneath the first and second upper machining stations. First and second spindles have respective workpiece grabs and are displaceable vertically on the frame between upper positions with the respective grabs in the respective machining stations and lower positions with the respective grabs in the respective transfer stations. A workpiece conveyor extends through the transfer stations for transporting workpieces into and out of the transfer stations so that the spindles can pick up unmachined workpieces from the respective transfer stations and set machined workpieces down in the respective transfer stations. A holder carrying tools is displaceable on the frame between the machining stations so that while a workpiece is being machined by one of the tools in one of the machining stations a workpiece can be loaded into or unloaded from the grab of the transfer station of the other of the machining stations.
Such a machining apparatus operates on the pendulum principle, machining the workpieces alternately in both spindles. While a workpiece is being machined in the one spindle, the other workpiece can be loaded. For short machining times, the pendulum principle is especially advantageous when the idle times are equal to or greater than the machining times.
EP 1 711 309 describes two workpiece spindles and two tool turrets comprising four numerically controlled axes for the purpose of reducing idle times. Loading is effected by means of two loading and unloading devices that are disposed on both sides of the work space.
Since however the chips of produced by such a machining apparatus operating on the pendulum principle are accumulated in alternating fashion at both spindles, they must stowed or shielded. Particularly in compact machines, there is only very little space for the chip guard. No matter what is done, some of them end up accumulating on the conveyors, entailing serious down time while everything is periodically cleaned.